Notes

Over the past century or so a variety of global properties of networks have been studied. Typical ones include:

• Edge connectivity: the minimum number of connections that must be removed to make the network disconnected.

• Diameter: the maximum distance between any two nodes in the network. The pictures below show the largest planar trivalent networks with diameters 1, 2 and 3, and the largest known ones with diameters 4, 5 and 6.

• Circumference: the length of the longest cycle in the network. Although difficult to determine in particular cases, many networks allow so-called Hamiltonian cycles that include every node. (Up to 8 nodes, all 8 trivalent networks have this property; up to 10 nodes 25 of 27 do.)

• Girth: the length of the shortest cycle in the network. The pictures below show the smallest trivalent networks with girths 3 through 8 (so-called cages). Girth can be relevant in seeing whether a particular cluster can ever occur in network.

• Chromatic number: the minimum of colors that can be assigned to nodes so that no adjacent nodes end up the same color. It follows from the Four-Color Theorem that the maximum for planar networks is 4. It turns out that for all trivalent networks the maximum is also 4, and is almost always 3.